Loading dataset throws "KeyError: 'Field "builder_name" does not exist in table schema'"
## Describe the bug
I have prepared dataset to datasets and now I am trying to load it back Finnish-NLP/voxpopuli_fi
I get "KeyError: 'Field "builder_name" does not exist in table schema'"

My dataset folder and files should be like @patrickvonplaten  has here https://huggingface.co/datasets/flax-community/german-common-voice-processed

How my voxpopuli dataset looks like:
![image](https://user-images.githubusercontent.com/25264037/144895598-b7d9ae91-b04a-4046-9f06-b71ff0824d13.png)

Part of the processing (path column is the absolute path to audio files)
```
def add_audio_column(example):
    example['audio'] = example['path']
    return example

voxpopuli = voxpopuli.map(add_audio_column)
voxpopuli.cast_column("audio", Audio())
voxpopuli["audio"] <-- to my knowledge this does load the local files and prepares those arrays
voxpopuli = voxpopuli.cast_column("audio", Audio(sampling_rate=16_000)) resampling 16kHz
```

I have then saved it to disk_
`voxpopuli.save_to_disk('/asr_disk/datasets_processed_new/voxpopuli')`

and made folder structure same as @patrickvonplaten 
I also get same error while trying to load_dataset from his repo:
![image](https://user-images.githubusercontent.com/25264037/144895872-e9b8f326-cf2b-46cf-9417-606a0ce14077.png)


## Steps to reproduce the bug
```python
dataset = load_dataset("Finnish-NLP/voxpopuli_fi")
```

## Expected results
Dataset is loaded correctly and looks like in the first picture

## Actual results
Loading throws keyError:
KeyError: 'Field "builder_name" does not exist in table schema'


Resources I have been trying to follow:
https://huggingface.co/docs/datasets/audio_process.html 
https://huggingface.co/docs/datasets/share_dataset.html

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.16.2.dev0
- Platform: Ubuntu 20.04.2 LTS
- Python version: 3.8.12
- PyArrow version: 6.0.1
